The present invention relates to the recovery of uranium from subterranean ore deposits and more particularly to an in-situ leaching employing an alkaline lixiviant with a hypochlorite oxidizing agent.
The various techniques for the production of uranium ore deposits may be characterized as falling within two general classes. One involves a surface milling operation in which uranium ore obtained by mining is crushed and blended and then subjected to a leaching procedure in which an acid or alkaline lixiviant is employed to extract uranium from the milled ore. The uranium is then recovered from the pregnant lixiviant by a suitable technique such as solvent extraction, direct precipitation, or by adsorption and elution employing an ion exchange resin. The other involves in-situ leaching in which a lixiviant is introduced into a subterranean ore deposit through a suitable injection system. The lixiviant may be an acidic or alkaline medium which solubilizes uranium values as it traverses the ore body. The pregnant lixiviant is then withdrawn from the ore body through a production system and treated to recover uranium therefrom by suitable techniques such as noted above. Mill leaching and in-situ leaching operations are similar in some respects and quite dissimilar in others. In both cases, the nature of the lixiviant is dictated to some extent by the nature of the uranium ore or the subterranean deposit. An acid lixiviant is used in most mill leaching operations since it is more effective with most ores and does not require that the ore be ground to as fine a state as in the case of an alkaline lixiviant. The use of acid lixiviant is somewhat limited in milled ores of high carbonate content which may lead to excessive consumption of acid. The presence of carbonate in subterranean rock deposits containing uranium also limits the use of acid lixiviants, not only with respect to acid consumption, but also due to the precipitation of reaction products, such as calcium sulfate which may result in plugging of the formation when sulfuric acid is used. Thus the use of an alkaline lixiviant is strongly indicated in many in-situ leaching operations, not only because of the carbonate content of the rock but also since alkaline lixiviants are more selective with respect to uranium dissolution than acid lixiviants.
In both milling and in-situ leaching operations, an oxidizing agent is employed in conjunction with the lixiviant in order to ensure that the uranium is oxidized to or retained in the hexavalent state at which it is solubilized by the acid or alkaline leach medium.
In milling operations employing an acid lixiviant, the oxidizing reaction requires the presence of iron and the principal oxidizing agents employed are manganese dioxide and sodium chlorate as disclosed in Merritt, Robert C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines, Research Institute, U.S.A. (1971), p. 63. In an alkaline leach system, the tetravalent uranium is directly oxidized to the hexavalent state. As disclosed by Merritt, pages 104 and 105, the most widely used oxidant in milling operations employing an alkaline lixiviant is potassium permanganate. Other oxidizing agents disclosed by Merritt include sodium hypochlorite, hydrogen peroxide, potassium persulfate and copper sulfate.
In in-situ leaching operations employing an alkaline lixiviant, the most commonly employed oxidizing agents are hydrogen peroxide as disclosed in U.S. Pat. No. 2,896,930 to Menke and air as disclosed in U.S. Pat. No. 2,954,218 to Dew et al. A significant distinction between mill leaching and in-situ leaching resides in the fact that the latter procedure is carried out in a massive subsurface formation where the uranium mineral is present in small concentrations without the intimate contact between the lixiviant and ore found in milling operations where the ore is rubblized before leaching. Thus a rapid intrinsic leaching rate is more important in in-situ leaching than in leaching carried out after a milling operation. Also many of the so-called refractory ores are much more difficult to leach in an in-situ environment than in conjunction with a milling operation. For example, as noted in the aforementioned patent to Dew et al., some uranium ore bodies contain carbonaceous material which retards the leaching action of the lixiviant. While such ores can be leached after milling, leaching under in-situ conditions is extremely slow without special procedures such as the in-situ combustion procedure disclosed in the Dew et al. patent.